Butyl rubber was first synthesized in 1937. It is the copolymer of isobutylene with a small amount of isoprene that is used as crosslinking site, and hence is also commonly known as isobutylene-isoprene rubber (IIR). Butyl rubber has excellent gas impermeability and chemical stability and so is widely used for inner tubes and inner liners of pneumatic tires. The heat resistance of butyl rubber renders its use for tire curing bags and bladders. Its resistance to ozone, weather, and moisture renders its use for roofing, reservoir membranes, electrical insulation, and automotive components. Isobutylene-isoprene rubber (IIR) has the general formula:
where x is an average number from 10 to 150; and n is an integer from about 100 to 10,000; and  reflects the cis/trans isomers centered on the adjacent double bond.
A viscoelastic rubber must be crosslinked to removes the translational mobility of the chains in order to become an elastic solid. Isoprene units in butyl rubber are the reactive sites for crosslinking. Because of the low concentration of unsaturated bonds, crosslinking of butyl rubber is slow or requires high temperature compared to highly unsaturated elastomers like natural rubber. This makes butyl rubber curing incompatible with curing of highly unsaturated rubbers such as natural rubber and styrene-butadiene rubber (SBR). There have been many attempts at making faster curing butyl rubber. For example, it was found that increasing the amount of isoprene units increased the curing rate. However, increasing the amount of isoprene in these systems was also found to depress the molecular weight of butyl rubber since the isoprene was a powerful chain transfer agent in polymerization.
Halogenated butyl rubbers developed in 1950's are very successful in this regard. Polymer 2 is an example of these halogenated butyl rubbers and has the formula:
where x is an average number from 50 to 100; n is an integer from about 100 to 1000; and  reflects the cis/trans isomers centered on the adjacent double bond. After halogenation of the isoprene units, the vulcanization rate significantly improves, rendering halobutyl rubbers better compatibility with highly unsaturated rubbers. As the result, halobutyl rubbers are the common choice for inner liners in tubeless tires.
Sulfur-accelerated vulcanization is the most popular method to cure butyl and halogenated butyl rubber. However, sulfur-cured vulcanization often produces materials that show poor high temperature properties due to the low dissociation energy of sulfidic crosslinks. These materials tend to soften when exposed to elevated temperatures of 300-400° F. for an extended period. In addition, these vulcanized products contain leachable and extractable residues, which has limited their application in pharmaceutical industry.
Carbon-carbon bonds are much more stable than the sulfur-sulfur bonds formed during vulcanization, giving better heat resistance. Peroxide is often used to introduce carbon-carbon bonds into elastomers to crosslink the elstomers. It is also known that these peroxide systems only generate small amounts of reaction byproducts. Unfortunately, however, in butyl rubber-peroxide systems, it has been found that the peroxide decomposes butyl rubber rather than crosslinking it.
Many attempts have been made to prepare a peroxide-curable butyl rubber, which could extend the application of butyl rubber. Chain scission efficiency of butyl rubber was found to decrease by increasing the unsaturation of butyl rubber, where butyl rubber could be crosslinked by peroxide when isoprene content was above 3 mol %. (See, Loan, L., The reaction between dicumyl peroxide and butyl rubbers. Journal of Polymer Science Part A: Polymer Chemistry 1964, 2 (5), 2127-2134, the disclosure of which is incorporated herein by reference in its entirety.) However, the molecular weight of butyl rubber was depressed by increasing the amount of isoprene units, which, as noted above, was a powerful chain transfer agent in polymerization. A terpolymer of isobutene, isoprene and divinylbenzene that can be crosslinked by peroxide has also been synthesized, but crosslinking in these systems happens during polymerization of the terpolymer, and they are difficult to process due to high gel fraction in the terpolymer.
Post-polymerization chemical modification is another way of crosslinking butyl rubbers that is often used in HIIR, owing to the higher reactivity of allylic halogen functionality compared to regular butyl rubber. In these systems ether, ester, ammonium or phosphonium functionalities are introduced into the HIIR by nucleophilic substitution reactions, in which an aliphatic alcohol, carboxylate salt, amine or phosphine are used as nucleophile, depending upon the functionality to be introduced. These elastomer derivatives contain pendent polymerizable functional groups such as styrenic, acrylic, maleimidic and vinylic functional groups, which act as crosslinking sites. The network forms rapidly due to high reactivity of these functional groups. These systems, however, are is limited by the allylic bromide content of starting material (bromobutyl rubber), have issues with chain scission, and do not produce materials having mechanical properties comparable with sulfur cured butyl rubbers. Another peroxide-curable butyl rubber has been synthesized via Suzuki-Miyaura coupling reaction of HIIR with 4-vinylphenylboronic acid and phenylboronic acid. The crosslinking density of resulting polymer in these systems is controlled by changing ratio of the two boronic acids. In these systems, however, material has to be purified for synthesis, since the acidic residue contained in material spoils the catalyst, which results in the low coupling efficiency. Similarly, epoxidized butyl rubbers have been synthesized by using m-chloroperoxybenzoic acid with regular butyl rubber. The ring-opening/elimination of epoxidized butyl rubber provides another way to prepare multifunctional graft copolymers, but does not produce materials having mechanical properties comparable with sulfur cured butyl rubbers. In other cases, maleic anhydride has been grafted on regular butyl rubber by using peroxide to improve its adhesion performance, but this approach was also found to cause decrease of molecular weight of the butyl rubber.
While, as set forth above, various derivatives of butyl rubber can be crosslinked by peroxide, the mechanical properties of peroxide cured butyl rubbers, such as tensile strength, strain at break, or toughness, are still inferior to those vulcanized by sulfur. Co-agents which contains multiple allylic, acrylic, or maleimide groups are frequently applied in peroxide system to increase crosslinking density of product. In HIIR-peroxide systems, for example, bismaleimide has often been used. These co-agent systems also fail to produce materials having mechanical properties comparable with sulfur cured butyl rubbers. A series of co-curing butyl rubber derivatives from BIIR have been synthesized. It has been found that co-curing elastomers bearing polyether, vinyl ether side chains provide good crosslinking yields due to their reactivity with N-arylmaleimides. But again, these systems fail to produce materials having mechanical properties comparable with sulfur cured butyl rubbers.
What is needed in the art is an efficient way to modify the isobutylene-isoprene rubber to produce a readily crosslinkable polyisobutylene-based rubber that avoids the use of corrosive bromine or chlorine to make the activated butyl rubber and is easier to crosslink than known halobutyls by, for example, crosslinking using a simple organic base or peroxide.